Many complex tasks, including those in military command and control settings, now involve the use of visual displays, such as computer displays. Individuals using these displays are required to make efficient searches of their screens to locate pertinent information, and the speed and accuracy with which they do so impacts their ability to carry out their responsibilities. To evaluate an individual's performance as well as a display's usefulness, it is considered desirable to know precisely where and for long an individual looks at the display during critical times. In addition, in assessing the effectiveness of any visual display, it is useful to know not only what features of the display an individual focuses on, but whether cognitive activity occurs.
It has been proposed that pupillary response is a physiological correlate of cognitive activity, in that the pupil dilates as the subject gazes at a point which stimulates cognition. This relationship is part of the underlying rationale of Kahneman's theory of attention (Kahneman, D., Attention and Effort, Prentice-Hall, Englewood Cliff, N.J. (1973)) and has been the focus of a number of studies involving different cognitive activities including: Mental arithmetic (Hess, E. H. and J. M. Polt, Science 140:1190-1192 (1964); Bradshaw, J. L., Quart. J. Expl. Psych. 20:116-122 (1968); Ahern, S. and J. Beatty, Science 205:1289-1292 (1979)), sentence processing (Metalis, S. A. et al., J. Applied Psych. 65:359-363 (1980); Schluroff, M., Brain and Language 17:133-145 (1982); Just, M. A. and P. A. Carpenter, Can. J. Expl. Psych. 47:310-339 (1993)), visual search (Backs, R. W. and L. C. Walrath, Appl. Ergonom. 23:243-254 (1992)), and digit span recall (Gardner, R. M., et al., Percep. Motor Skills 41:951-955 (1975); Granhohn, E. et al., Psychophysiol. 33:457-461 (1996).
Pupil dilation is primarily the result of the integrated activity of two groups of muscles located in the iris. One set of muscles (the circular muscles) encircles the pupil; when activated, this set serves to constrict the diameter of the pupil and make it smaller. The second set of muscles (the radial muscles) lies immediately outside the circular muscles and extends radially from the pupil out through the iris. When activated, the radial muscles pull the pupil diameter outward and cause it to become larger. These two sets of muscles (the radial and the circular) typically work together through reciprocal innervation, a physiological process involving both agonistic and antagonistic responses.
In the presence of steady light, an individual's pupil responds with a continual but irregular oscillation. This movement is known as the light reflex, and it has been extensively studied. During the light reflex, the circular muscles act as the agonist and are stimulated to contract, while the radial muscles act as the antagonist and are inhibited from dilating the pupil. The reflex is fleeting, and the result is a visible pulsing of the pupil. This movement does not appear to be tied to other physiological systems such as respiration or heart rate (Lowenfeld, I. E., The pupil: Anatomy, physiology, and clinical applications (Volume I) Iowa State University Press, Ames, Iowa and Wayne State University Press, Detroit, Mich. (1993)).
When the individual experiences a psychosensory stimulus, e.g. a task requiring significant cognitive processing, the pupil may make a response that is quite different from the light reflex as the process of reciprocal innervation is reversed: The radial muscles are activated (causing the pupil to dilate), and the circular muscles are inhibited (also causing the pupil to dilate). The result is a brief dilation that is greater than either muscle group alone could effect. For this reason, the phenomenon is called the dilation reflex (Loewenfeld, 1993). Like the light reflex, the dilation reflex is a transitory event. In both instances, one observes a pulsing of the diameter, but with the dilation reflex the pulses are irregular and sharp, often exhibiting large jumps followed by rapid declines.
The fundamental problem in studying the relationship between cognitive activity and pupillary response lies in how to separate the dilation reflex from the light reflex. It is the dilation reflex that is important to measure, because this is the manifestation of cognitive workload (Loewenfeld, 1993). The dilemma is that both phenomena (dilation reflex and light reflex) may occur at the same time. Indeed, most cognitive tasks are carried out in lighted situations, which means that the light reflex will be present. Thus, it is considered desirable to develop a technique that can identify and remove the light reflex (essentially background noise) to reveal the dilation reflex that accompanies cognitive activity.
Two recurring themes have emerged from the cognitively-oriented research on pupil dilation: The pupil dilates as a result of effortful cognitive processing, and the degree of dilation varies with the degree of difficulty of the task. However, heretofore it has not been possible to look across tasks and compare the level or pattern of observed dilation because there is as yet no standard approach to measuring pupil dilation. Some researchers have adopted an averaging procedure; others have implemented a simple difference between adjacent observations; and others have used smoothing techniques from signal processing. Baseline measures differ from study to study, and virtually all reported data have been subjected to substantial and not always well-specified "preprocessing" of the data in order to produce meaningful representations. In short, researchers in this area have no unifying basis for comparisons either across tasks or across individuals as they attempt to understand the relationship between pupil dilation and cognitive processing.